A mutation is a permanent change in the sequence of bases in DNA.
Review Questions
Translate the following segment of DNA into RNA: AGTTC
Write the complimentary DNA nucleotides to this strand of DNA: GGTCCA
What makes up a nucleotide?
Nucleotides are subunits of which polymers?
Amino acids are subunits that make up what polymer?
Describe the process of DNA replication
Name a mutagen.
What is made in the process of transcription?
What is made in the process of translation?
How does RNA encode for proteins?
Further Reading / Supplemental Links
http://www.phschool.com/science/biology_place/biocoach/dnarep/intro.html
http://nobelprize.org/educational_games/medicine/dna_double_helix/readmore.html
http://www.biostudio.com/demo_freeman_protein_synthesis.htm
http://learn.genetics.utah.edu/units/basics/transcribe/
http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html
http://learn.genetics.utah.edu/units/basics/builddna/
http://enwikipedia.org/
http://sickle.bwh.harvard.edu/scd_background.html
Vocabulary
amino acid
The units (monomers)that combine to make proteins.
DNA
Deoxyribonucleic acid; a nucleic acid that is the genetic material of all organisms.
DNA replication
The synthesis of new DNA; occurs during the S phase of the cell cycle.
double helix
Describes the shape of DNA as a double spiral; similar to a spiral staircase.
gene
The inherited unit of DNA that encodes for one protein (or one polypeptide).
mutagen
A chemical or physical agent that can cause changes to accumulate in DNA.
mutation
A change in the nucleotide sequence of DNA.
nucleotide
The units that make up DNA; consists of a 5-carbon sugar, a phosphate group, and a nitrogen-containing base
RNA
The nucleic acid that carries the information stored in DNA to the ribosome.
semiconservative replication
Describes how the replication of DNA results in two molecules of DNA, each with one original strand and one new strand.
transcription
The synthesis of a RNA that carries the information encoded in the DNA.
translation
The synthesis of proteins as the ribosome reads each codon in RNA, which code for a specific amino acid.
Points to Consider
Your cells have “proofreaders” that replace mismatched pairs that occurred during DNA synthesis. How would that affect the rate of mutation in your body?
There are many diseases due to mutations in the DNA. These are known as genetic diseases, and many can be passed onto the next generation. Think about how a single base change cause a huge medical problem like sickle cell anemia.
Your DNA contains the instructions to make you. So is everyone’s DNA different? Can it be used to distinguish individuals, like a fingerprint?
Chapter 6: Genetics
Lesson 6.1: Gregor Mendel and the Foundations of Genetics
Lesson Objectives
Explain Mendel's law of segregation.
Draw a Punnett square to make predictions about the traits of the offspring of a simple genetic cross.
Check Your Understanding
What is the genetic material of our cells?
How does meiosis affect the chromosome number in gametes?
Introduction
For centuries people have been fascinated with the inheritance of human traits. People might say, “You have your father’s eyes.” or, “You have red hair like your granddad; it must have skipped a generation.” These comments show an appreciation of the laws of human inheritance. We inherit traits from our ancestors, and sometimes traits can stay hidden and show up in later generations. Genetics, the study of inheritance, explains how traits are passed on from one generation to the next. Due to recent developments in the field of genetics, we can now seek to understand the inheritance of disease. People today may ask “What are the chances my child will have cystic fibrosis?” and “What is the likelihood that I may have breast cancer if my grandmother had it?” Genetic counselors are trained to address families’ questions about the probabilities of passing on a genetic disorder. When genetic counselors sit down with families to discuss these types of questions, it’s amazing that their answers are derived from the fundamentals of genetics discovered by a monk in the 1800s.
Figure 6.1
Gregor Mendel
Mendel’s Experiments
The laws of heredity were first developed by an Austrian monk, Gregor Mendel (Figure above), in the 1800s. To study genetics, Mendel chose to work with pea plants because they had easily observable traits and a short generation time (Figure below). For example, pea plants are either tall or short, which are easily identifiable traits. Furthermore, peas can either self pollinate or be cross-pollinated by hand, by transferring the pollen from one flower to the stigma of another. In this way, Mendel could carefully observe the results of crosses between two different types of plants. He studied the inheritance patterns for many different traits in peas, including round seeds versus wrinkled seeds, white flowers versus purple flowers, and tall plants versus short plants. Because of his work, Mendel can be considered the father of genetics.
Figure 6.2
Characteristics of pea plants.
During Mendel’s time, most people believed that traits were contributed from both parents and blended together as they were passed down from generation to generation. For example, if you crossed a short plant and a tall plant, they would expect the offspring to be medium-sized plants. What Mendel observed, however, was that the offspring of this cross (called the F1 generation, derived from the Latin term filius, meaning sons and daughters) were all tall plants. Based on the blending hypothesis, the result of all tall plants was unexpected.
Next, Mendel let the F1 generation self-pollinate. He then noted that 75% of the resulting offspring, the F2 generation, were tall, while 25% were short. Therefore, shortness appeared to have skipped a generation. Mendel found this same mathematical result over and over again with all the traits he studied. In all, Mendel studied seven characteristics, with almost 20,000 F2 plants analyzed. For example, purple flowers and white flowers were crossed to produce plants with only purple flowers in the F1 generation. Then after self-pollination, the F2 generation had 75% purple flowers and 25% white flowers. These results did not reflect what you would expect if the blending model of inheritance was correct.
Dominance
Mendel had to come up with a new theory of inheritance to explain his results. His explanation, the law of segregation, is still one of the fundamental laws of modern genetics. He proposed that each pea plant had two hereditary factors for each trait. There were two possibilities for each hereditary factor, such as short or tall. One factor is dominant to the other, meaning it masks the effects of the recessive factor. However, each parent could only pass on one of these factors to the offspring. Therefore, during the formation of gametes, sperm or egg, the heredity factors must separate so there is only one factor per gamete. When fertilization occurs, the offspring then have two hereditary factors.
This law explained what Mendel had seen in the F1 generation, because the two heredity factors were the short and tall factors and each individual in the F1 would have one of each factor, and as the tall factor is dominant to the short factor, all the plants appeared tall. In the F2 generation, produced by self-pollination of the F1, 25% of the offspring could have two short heredity factors, so they would appear short. 75% would have at least one tall heredity factor and will be tall.
In genetics problems, the dominant factor is labeled with
a capital letter (T) while the recessive factor is labeled with a lowercase letter (t). If we designate the letter T or t to represent the heritable factor, as each individual has two factors for each trait, the possible combinations are Tt, TT, and tt. Plants with TT would be tall while plants with tt would be short. Since T is dominant to t, plants that are Tt would be tall, as with the F1 generation we described that inherited one factor from the TT tall parent and one factor from the tt short parent.
Probability and Punnett Squares
To visualize the results of a genetic cross, a Punnett square is helpful. An example of a Punnett square (Figure below) that shows the results of a cross between two purple flowers that each have one dominant factor and one recessive factor (Bb). Notice how the possible factors in the sperm (B or b) are lined up the side of the square while the possible factors in the egg (B or b) are lined up across the top. The possible offspring are represented by the letters in the boxes, with one factor coming from each parent.
Notice how the Punnett square can help you predict the outcome of the crosses. Only one of the plants out of the four, or 25% of the plants, has white flowers (bb). The other 75% have purple flowers (BB, Bb) because the color purple is a dominant trait in pea plants.
Now imagine you cross one of the white flowers (bb) with a purple flower that has both a dominant and recessive factor (Bb). The only possible gamete in the white flower is the recessive (b), while the purple flower can have gametes with either dominant (B) or recessive (b). If you write out the Punnett cross, you will see that 50% of the offspring will be purple and 50% of the offspring will be white.
Keep in mind that the birth of each offspring is an independent event and has the same probability, so the traits of a previous offspring do not influence the next offspring. In the cross discussed above with two Bb flowers, each offspring has a 75% chance of being purple and a 25% chance of being white. For example, even if the first three offspring in the cross have purple flowers, it does not mean that the next plant must have white flowers. All probability tells you is that overtime the averages of many, many offspring will work out to a predicted ratio.
The Punnett Square of a white flower (bb) crossed with a purple flower (Bb) b b
B Bb bb
b Bb bb
Figure 6.3
The Punnett Square of a cross between two purple flowers (Bb)
Lesson Summary
Gregor Mendel is considered the father of genetics, the science of studying inheritance.
According to Mendel’s law of segregation, an organism has two factors for each trait, but each gamete only contains one of these factors.
A Punnett square is useful for predicting the outcomes of crosses.
Review Questions
What is the term for the offspring of a cross, or the first generation?
What is the F2 generation?
Who is considered the father of genetics?
Why did Mendel select peas as a model for studying genetics?
In peas, yellow seeds (Y) are dominant over green seeds (y). If a yy plant is crossed with a YY plant, what ratio of plants in the offspring would you predict?
In peas, purple flowers (P) are dominant over white flowers (p). If a pp plant is crossed with a Pp plant, what ratio of plants in the offspring would you predict?
In guinea pigs, black fur (B) is dominant over white fur (b). If a BB guinea pig is crossed with a Bb guinea pig, what ratio of guinea pigs in the offspring would you predict?
In guinea pigs, smooth coat (S) is dominant over rough coat (s). If a SS guinea pig is crossed with a ss guinea pig, what ratio of guinea pigs in the offspring would you predict?
In humans, unattached ear lobes are dominant over attached ear lobes. If two parents have attached earlobes, what is the predicted ratio in the offspring?
Why would it be much easier to study genetics in pea plants than in people?
Further Reading / Supplemental Links
http://www.mendelweb.org/MWtoc.html
http://www.estrellamountain.edu/faculty/farabee/BIOBK/BioBookgenintro.html
http://sonic.net/~nbs/projects/anthro201/
http://anthro.palomar.edu/mendel/mendel_1.htm
Vocabulary
dominant
Masks the expression of the recessive trait.
F1 generation
The first filial generation; offspring of the P or parental generation.
F2 generation
The second filial generation; offspring from the self-pollination of the F1 generation.
gametes
Haploid cells involved in sexual reproduction, such egg and sperm.
genetics
The study of inheritance.
Punnett square
Visual representation of a genetic cross that helps predict the expected ratios in the offspring, first described by Reginald C. Punnett in the early 20th century.
recessive
Expression is masked by the dominant factor (allele); only expressed if both factors are recessive.
Points to Consider
Do you think all traits follow this simple pattern where one factor controls the trait?
Can you think of other examples where Mendel’s law does not seem to fit?
Lesson 6.2: Modern Genetics
Lesson Objectives
Explain Mendel’s laws with our modern understanding of chromosomes.
Explain how codominant traits are inherited.
Distinguish between phenotype and genotype.
Explain how polygenic traits are inherited.
Check Your Understanding
What is a visual representation of a genetic cross?
What is stated in Mendel’s law of segregation?
Introduction
Although Mendel laid the foundation for modern genetics, there were still a lot of questions left unanswered. How is inheritance determined for traits that do not seem to follow a simple dominant-recessive pattern? What exactly are the hereditary factors that determine traits in organisms? And how do these factors work? One of the great achievements of this past century was the discovery of DNA as the genetic material. And it is the DNA that makes up the hereditary factors that Mendel identified. By applying our modern knowledge of DNA and chromosomes, we can explain Mendel’s findings and build on them.
Traits, Genes, and Alleles
Interpreting Mendel’s discoveries through the eye of modern genetics, we now know that Mendel’s hereditary factors are made up of DNA. Recall that our DNA is wound into chromosomes. Each of our chromosomes contains a long chain of DNA that encodes hundreds, if not thousands, of genes. Each of these genes can have slightly different versions from individual to individual. These variants of genes are called alleles. For example, remember that for the height gene in pea plants there are two possible alleles, the dominant allele for tallness (T) and the recessive allele for shortness (t).
Genotype and Phenotype
Genotype refers to the combination of alleles that an individual has for a certain gene. For each gene, an organism has two alleles, one on each chromosome of a homologous pair of chromosomes. The genotype is often referred to with the letter combinations that were introduced in the previous lesson, such as TT, Tt, and tt. When an organism has two of the same alleles for a specific gene, it is homozygous for that gene. An organism can be either homozygous dominant (TT) or homozygous recessive (tt). If an organism has two different alleles (Tt) for a certain gene, it is known as heterozygous. Genes have a specific place on a specific chromosome, so in the heterozygous individual these alleles are in the same location on each homologous chromosome.
Phenotype refers to the visible traits or appearance of the organism, as determined by the genotype. For example, the phenotypes of Mendel’s pea plants were either tall or short, or were purple-flowered or white-flowered. Keep in mind that plants with different genotypes can have the same phenotype. For example, both a pea plant that is homozygous dominant for the tall trait (TT) and heterozyg
ous plant (Tt) would have the phenotype of being tall plants. The recessive phenotype only occurs if the dominant allele is absent, which is when an individual is homozygous recessive (tt).
Incomplete Dominance and Codominance
In all of Mendel’s experiments, he worked with traits where a single gene controlled the trait and where one allele was always dominant to the other. Although the rules that Mendel derived from his experiments explain many inheritance patterns, the rules do not explain them all. There are in fact exceptions to Mendel’s rules, and these exceptions usually have something to do with the dominant allele.
One exception to Mendel’s rules is that one allele is always completely dominant over a recessive allele. Sometimes an individual has an intermediate phenotype between the two parents, as there is no dominant allele. This pattern of inheritance is called incomplete dominance.
An example of incomplete dominance is the color of snapdragon flowers. One of the genes for flower color in snapdragons has two alleles, one for red flowers and one for white flowers. A plant that is homozygous for the red allele will have red flowers, while a plant that is homozygous for the white allele will have white flowers. On the other hand, the heterozygote will have pink flowers (Figure below). Neither the red nor the white allele is dominant, so the phenotype of the offspring is a blend of the two parents.
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